Exemplary embodiments of the invention relate to a method for operating an internal combustion engine, and an internal combustion engine which is designed for carrying out the method.
In automotive technology it is customary to filter soot particles from the exhaust gas, in particular from diesel engines, by means of a particle filter. The aim is to avoid an excessive accumulation of filtered particles in the particle filter. Accumulated soot particles in the particle filter may be continuously removed by oxidation, using NO2 and H2O as oxidizing agents, at temperatures above approximately 250° C. This effect is also known as the continuous regeneration trap (CRT) effect, referring to a filter having continuous regeneration. Since the nitrogen oxides contained in the exhaust gas usually consist almost exclusively of NO, it is customary to increase the NO2 portion by oxidizing this NO in an oxidation catalytic converter upstream from the particle filter. A corresponding system made up of a diesel oxidation catalytic converter (DOC) and a diesel particle filter (DPF) is known as a CRT System™.
The formation of NO2 at the oxidation catalytic converter is improved by high proportions of noble metals, for example platinum and platinum group metals, which entails corresponding costs and is therefore disadvantageous. If the soot removal rate achieved by the CRT effect is not adequate, the particle filter is actively regenerated by thermally burning off the soot, specifically, by means of the oxygen contained in the exhaust gas. However, this thermal regeneration requires much higher temperatures of typically greater than 550° C. Such high temperatures seldom occur during normal driving operation, in particular in diesel engines, and must therefore often be provided by measures internal and/or external to the engine. This may take place, for example, by enrichment of exhaust gas with fuel, in particular in the form of uncombusted or partially combusted fuel, by means of fuel post-injection in the engine and subsequent exothermic oxidation of this fuel in an oxidation catalytic converter situated upstream from the particle filter and/or in the particle filter itself. The heating of the particle filter may be influenced by suitably adjusting the post-injection parameters, such as the time and quantity of the post-injection. Numerous variation options are described, for example, in PCT Publication WO 2005/003537 A1.
Exemplary embodiments of the invention are directed to an operating method for an internal combustion engine that effectively assists in particle filter regeneration, even under unfavorable operating conditions. Moreover, exemplary embodiments of the invention are directed to an internal combustion engine in which particle filter regeneration is made possible in the widest possible operating range, in particular even under partial load or low load.
In the method according to the invention, for assisting a regeneration of a particle filter situated in an exhaust gas tract of the internal combustion engine and downstream from an oxidation catalytic converter, post-injections of fuel by means of an injector take place in at least one cylinder of the internal combustion engine. According to the invention, it is additionally provided to advance a closing time of an exhaust valve of a cylinder of the internal combustion engine. The advance of the closing time is carried out in a first temperature range for a temperature of the oxidation catalytic converter, and the post-injections take place in a second temperature range for the temperature of the oxidation catalytic converter, an upper range limit of the first temperature range having a lower value than an upper range limit of the second temperature range.
The invention is based on the finding that, for carrying out intermittently performed particle filter regeneration by thermal soot burn-off, as well as for carrying out continuously performed particle filter regeneration by soot oxidation by means of NO2, in particular at low loads of the internal combustion engine it may be necessary to raise the exhaust gas temperature to a correspondingly required level. However, fuel post-injection has been found to be disadvantageous in two respects. First, oxidation of fuel introduced into the exhaust gas by post-injection cannot take place when the temperature of the oxidation catalytic converter is below its light-off temperature, i.e., the oxidation catalytic converter is not yet able to oxidize the fuel, which is present in the form of uncombusted or partially combusted fuel, to an appreciable extent. Second, it has been shown that uncombusted or partially combusted fuel portions in the exhaust gas may inhibit the ability of the oxidation catalytic converter to oxidize NO to NO2, in particular at comparatively low temperatures. These disadvantages may be avoided by carrying out advancement of the exhaust valve closing time, which increases the temperature, in an essentially lower temperature range that is the same as for the post-injection. Starting from low temperatures, heating of the exhaust gas initially takes place by advancing the exhaust valve control times. After the oxidation catalytic converter has reached a suitable temperature level, further heating takes place by oxidation of fuel that is introduced into the exhaust gas via the post-injection.
The post-injection preferably takes place in a range of approximately 60° crank angle to approximately 170° crank angle, particularly preferably in a range of 90° crank angle to 150° crank angle after top dead center, in the power stroke in at least one cylinder. At best, this results in a negligible influence of the post-injection on the torque provided by the internal combustion engine. Essentially uncombusted, possibly cracked, fuel is then discharged from the cylinder in question. This fuel is composed predominantly of short- to medium-chain hydrocarbons, carbon monoxide, and hydrogen. When post-injection takes place, at least one such post-injection is usually carried out after each torque-effective main injection.
Up to five individual post-injections per power stroke of the internal combustion engine operating according to the four-stroke process may take place in a time window having a duration of approximately 20 ms. In particular, one or more injections may be performed over a period of 0.2 ms to 3 ms in the power stroke. This period of 0.2 ms to 3 ms, for example, thus represents the speed-dependent time window per power stroke or working cycle of a particular cylinder operated with post-injection.
The second temperature range, i.e., the temperature range in which the post-injections are provided, preferably extends approximately from a lower range limit up to the upper range limit in the range of the light-off temperature of the oxidation catalytic converter, which may be regarded as the optimal temperature for formation of the CRT effect, and which is approximately 450° C. for the oxidation catalytic converter. The light-off temperature of a catalytic converter is usually understood to mean the temperature above which a noticeable quantity, typically approximately 50%, of the exhaust gas constituents to be treated is converted. A typical value of the light-off temperature is approximately 230° C. to 250° C.
At higher temperatures in the vicinity of the upper range limit of the second temperature range, the downstream particle filter is also advantageously heated by heat transfer. However, heating of the oxidation catalytic converter due to the exhaust gas enrichment with fuel by means of post-injection within the engine, resulting in an exhaust gas temperature downstream from the oxidation catalytic converter of greater than 330° C., and thus, a particle filter temperature of likewise greater than 330° C., may be quite adequate. The upper range limit of the second temperature range is typically not above 480° C. for the oxidation catalytic converter or for exhaust gas temperatures present on the outlet side of the oxidation catalytic converter, in order to minimize the complexity to the greatest extent possible.
Particularly effective oxidation of nitrogen oxide (NO) contained in the exhaust gas to nitrogen dioxide (NO2), and particularly effective soot oxidation by the NO2 that is generated, are made possible due to the setting of the temperature of the oxidation catalytic converter brought about by the post-injection. This in turn allows use of an oxidation catalytic converter having reduced loading of the noble metals platinum and/or palladium. In addition, particle filter regeneration by thermal soot burn-off may be largely avoided. The thermal load on the oxidation catalytic converter and disadvantageous aging effects are thus minimized. Therefore, little or no noble metal has to be kept on hand for compensating for an aging-related drop in activity.
Therefore, in the present case an oxidation catalytic converter is preferably used whose noble metal content, based on a volume of the oxidation catalytic converter, is in the range of 170 g/m3 to 700 g/m3, in particular approximately 350 g/m3. Noble metal loadings of catalytic converters are also expressed in g/ft3; with these units, the above-mentioned quantities correspond to values of 5 g/ft3 to 20 g/ft3, preferably 10 g/ft3. Such noble metal loadings are relatively low compared to typical values, which are in the range of approximately 40 g/ft3 to 70 g/ft3. The noble metals may comprise platinum and/or palladium. Even with such a low noble metal loading of the oxidation catalytic converter compared to customary values, significant activity for oxidation from NO to NO2 may be achieved in the present case. By use of an oxidation catalytic converter having the above-described comparatively low noble metal content, a significant soot conversion rate, based on the CRT reaction, may already be achieved at a temperature above approximately 230° C.
With regard to the advancing of a closing time of an exhaust valve of a cylinder, this causes the exhaust valve to close before an exhaust stroke of the particular cylinder has ended, resulting in intermediate compression of the exhaust gas in the cylinder. During this intermediate compression, compression work is performed that results in an increase in the temperature of the exhaust gas. The oxidation catalytic converter may thus be heated and in particular brought to its light-off temperature in a particularly easy manner. It is particularly preferred to advance the closing time of at least one exhaust valve in all cylinders of the internal combustion engine.
Advancing the exhaust valve closing time may take place by means of a so-called phase shifter, i.e., a variable camshaft adjustment. The phase shifter thus ensures that the exhaust valve of the corresponding cylinder of the internal combustion engine closes earlier, and in customary designs of the phase shifter, thus also opens earlier.
Advancing the exhaust valve closing time represents a very effective method for exhaust gas heating, even when a post-injection is ineffective in this regard because the oxidation catalytic converter is still below its light-off temperature. The first temperature range, in which the advancement takes place, therefore preferably extends from very low temperatures of approximately 0° C. or even lower, to an upper range limit in the range of the light-off temperature of the oxidation catalytic converter.
In one embodiment of the invention, the lower range limit of the second temperature range is selected in such a way that it at least approximately corresponds to the light-off temperature of the oxidation catalytic converter. Thus, enrichment of the exhaust gas with fuel as a result of the post-injection does not occur until the oxidation catalytic converter has been brought to its light-off temperature. It is thus ensured that the oxidation catalytic converter is able to largely react the fuel introduced into the exhaust gas, and thus be brought to a temperature that is suitable for providing NO2. In particular, it may be provided that exhaust gas enrichment or fuel post-injection is not activated until a certain threshold value or trigger value of the temperature of the oxidation catalytic converter of approximately 230° C., for example, as the lower range limit for the second temperature range, has been reached. It is generally preferred for fuel enrichment of the exhaust gas to take place in a temperature range of the oxidation catalytic converter of 250° C. to 450° C. This ensures that, although the oxidation catalytic converter has reached its light-off temperature, the temperature remains in a range in which thermal damage to the oxidation catalytic converter may be largely avoided.
In another embodiment of the invention, the upper range limit of the first temperature range falls within the second temperature range, or at least approximately coincides with the lower range limit of the second temperature range.
In the first case, a gradual transition occurs between the operation of the internal combustion engine with advanced control time of the exhaust valve and the operation in which post-injection is carried out. Thus, for example, in the range of the light-off temperature of the oxidation catalytic converter, a post-injection quantity of fuel introduced into a particular cylinder of the internal combustion engine is increased with increasing temperature, while at the same time, an advance of the closing time of the exhaust valve or the exhaust valves is decreased, in particular in comparison to the operation of the oxidation catalytic converter strictly in heating mode. Thus, at the beginning, when the exhaust valve is still opening comparatively early, it is possible for only a small quantity of fuel to be introduced into the second cylinder via the post-injection. The farther the exhaust valve is advanced toward an early closing or opening time, the larger the quantity of fuel that can then be introduced into the cylinder in each post-injection. Wetting of the cylinder wall with post-injected fuel is thus largely avoided.
Undesirable wetting of the cylinder wall may result when, for early closing of the exhaust valve, the exhaust valve is also opened comparatively early. In this case, the pressure in the combustion chamber drops during the power stroke of the particular cylinder. If a post-injection takes place at this point in time, there is a risk of wetting the cylinder wall with fuel. This is due to the fact that, with reduced pressure in the combustion chamber, the fuel jet has a particularly large range during the post-injection. Wetting of the cylinder wall may in turn result in undesirable dilution of the engine lubricating oil. To prevent this, it may be provided that when post-injections are carried out, the control times of the exhaust valve are not advanced, and conversely, the closing or opening time of the exhaust valve is advanced only when post-injection is (still) not being carried out. Thus, post-injections are not begun until the control times of the exhaust valve once again correspond to those of normal operation without intermediate compression. In this case, the upper range limit of the first temperature range coincides with the lower range limit of the second temperature range.
In another embodiment of the invention, within the first temperature range, essentially a reduction in the advance of the closing time of the exhaust valve takes place with increasing temperature of the oxidation catalytic converter. Preferably the advance is progressively reduced with increasing temperature, in particular to quickly reset it to zero in the range of the light-off temperature or just before reaching same, and thus to produce the normal operating state. Further heating of the oxidation catalytic converter then occurs by oxidation of post-injected fuel.
In another embodiment of the invention, the post-injections in at least one of the cylinders of the internal combustion engine take place in a clocked manner, such that first time periods of a predefinable duration, in which post-injections are carried out in each working cycle of a particular cylinder, alternate in direct succession with second time periods of predefinable duration, in which the post-injections are prevented.
Depending on the heating requirement, a fairly large number of alternating first time periods that follow one another in direct succession are provided with exhaust gas enrichment or fuel post-injection in at least one cylinder, and second time periods are provided without exhaust gas enrichment or fuel post-injection. Due to this pulsed fuel enrichment of the exhaust gas, and carrying out a post-injection only intermittently in at least one cylinder, it is possible to achieve particularly high NO2 formation at the oxidation catalytic converter and a particularly high oxidation rate by NO2 and H2O for soot deposited in the particle filter. The CRT effect is thus particularly strong. This is because, among other things, during fuel enrichment of the exhaust gas, i.e., during the first time period, the oxidation catalytic converter is brought to a temperature that allows the highest possible activity of same with regard to oxidation of NO contained in the exhaust gas to NO2, which may then also take place in the second time period unhindered and without inhibition of the oxidation catalytic converter by the CO and/or HC. The pulsed exhaust gas enrichment with fuel by post-injection within the engine, in which phases or first time periods with exhaust gas enrichment alternate with phases or second time periods without exhaust gas enrichment, thus results in effective passive soot oxidation of the particle filter, i.e., effective passive regeneration of the particle filter.
In the first time periods with exhaust gas enrichment or fuel post-injection, the oxidation catalytic converter, the exhaust gas, and the particle filter downstream from the oxidation catalytic converter are heated due to the exothermic oxidation of uncombusted and/or partially combusted fuel in the oxidation catalytic converter. In phases without exhaust gas enrichment or post-injection, NO2 formation and soot oxidation occur due to the CRT effect. In the process, the oxidation catalytic converter and the particle filter cool down only slightly on account of their heat capacity. The temperature to be provided for regenerating the particle filter may thus be significantly lowered in comparison to an active regeneration with thermal soot burn-off by oxidation with oxygen. This is accompanied by a reduction in the thermal load on the oxidation catalytic converter and the particle filter, which in particular may be a coated, i.e., catalytically active, particle filter.
Due to the low temperature load, aging of catalytic converters installed in the exhaust gas system, such as the oxidation catalytic converter and the particle filter, and optionally a selective catalytic reduction (SCR) catalytic converter or an ammonia slip catalytic converter, is reduced. This allows a significant reduction in the noble metal loading of the oxidation catalytic converter and/or of the particle filter. The SCR catalytic converter and/or the ammonia slip catalytic converter, which oxidizes ammonia possibly escaping from the SCR, may have correspondingly small dimensions. In addition, particularly inexpensive materials may be used for the SCR catalytic converter.
In the second time period, in which the exhaust gas is typically free of uncombusted fuel and comparatively rich in oxygen, the catalyst surface of the oxidation catalytic converter is cleaned of absorbed, inhibitive fuel molecules. As a result, the availability and the catalytically active surface of the oxidation catalytic converter are increased.
Thus, as a whole, all engine components or components of the exhaust gas system are cleaned by being intermittently acted on by exhaust gas enriched with fuel. Due to the cleaning and decontamination of the oxidation catalytic converter, a lowering of the light-off temperature of the oxidation catalytic converter is also achieved. In addition, the temporal separation of the heating, i.e., the enrichment of exhaust gas or the post-injection of fuel during the first time period, from the passive regeneration during the second time period results in a particularly good passive soot burn-off rate.
In another particularly advantageous embodiment of the invention the duration of the first time period and/or the duration of the second time period is/are predefined as a function of the temperature of the oxidation catalytic converter. The temperature may be measured downstream from the oxidation catalytic converter.
When the oxidation catalytic converter is comparatively cold, fuel may be introduced into at least one cylinder over a longer first time period than for when the oxidation catalytic converter is still comparatively hot. The first time period itself, i.e., the time period in which exhaust gas enrichment or post-injection is carried out anyway, is preferably in a range of 1 s to 300 s, in particular 3 s to 30 s. The duration of the second time period, during which the enrichment with fuel or post-injection of fuel in a second cylinder is prevented, may be in the range of 0.5 s to 200 s, in particular in a range of 10 s to 60 s. This second time period or post-injection pause is variable, and its duration in principle is also independent of the duration of the first time period. Thus, a shortening or lengthening of the first time period is not necessarily accompanied by a shortening or lengthening of the second time period. With the durations of the time periods described above, it is possible to achieve particularly extensive passive regeneration of the particle filter by means of the CRT effect.
When the oxidation catalytic converter is comparatively cold, and is heated only slightly above its light-off temperature, it is also advantageous to provide no, or only a very brief, post-injection pause, whereas when the oxidation catalytic converter is already comparatively hot, a correspondingly long post-injection pause may be provided. In particular, it is possible to provide no, or at best only a very brief, post-injection pause in a temperature window of 300° C. to 350° C. of the temperature of the oxidation catalytic converter or downstream therefrom.
Furthermore, a duration during which individual fuel post-injections are carried out in the at least one cylinder for each power stroke may be set as a function of the temperature. Such a short duration for each power stroke is particularly advantageous when the temperature is already high. Thus, namely the high temperature is easily maintained, and yet wetting of the wall of the at least one second cylinder with fuel is in particular largely prevented. In contrast, to achieve a comparatively large increase in the temperature of the oxidation catalytic converter, a longer overall duration of the post-injections for each power stroke may be provided, over which at least one post-injection is carried out in the at least one cylinder. This results in large injection quantities, which lead to a correspondingly rapid increase in the temperature.
In another embodiment of the invention, a divided recirculation of exhaust gas of the cylinders of the internal combustion engine into a feed air tract of the internal combustion engine is provided in such a way that exhaust gas only from a first portion of the cylinders is suppliable in an appreciable quantity to the feed air tract of the internal combustion engine, and exhaust gas from the remaining second portion of the cylinders is supplied essentially completely to the oxidation catalytic converter, with no recirculation, wherein post-injections of fuel take place only in at least one of the cylinders of the second portion of the cylinders. In other words, exhaust gas is enriched only for one or more cylinders whose exhaust gas is not recirculated. Stated another way, in general there is no fuel enrichment of exhaust gas by means of post-injection for one or more cylinders whose exhaust gas is recirculated.
This is based on the finding that fuel enrichment of the exhaust gas with uncombusted and/or partially combusted fuel may result in undesirable so-called sooting of the feed air tract when this exhaust gas is recirculated. This is avoided in the present case, since enrichment of exhaust gas is carried out only for the cylinder/cylinders whose exhaust gas is not able, or at best is able only to a negligible extent, to pass into an exhaust gas recirculation line via which exhaust gas is recirculatable into the feed air tract of the internal combustion engine. Functionally reliable operation of the internal combustion engine may be ensured in a particularly simple manner due to the avoidance of uncombusted fuel acting on the feed air tract, and in particular on a charge air cooler situated in the feed air tract or intake air path.
In another advantageous embodiment of the invention, for carrying out the post-injections, a control current of the injector is adjusted for actuating a valve needle of the injector as a function of a temperature of the oxidation catalytic converter. Changing the control current allows particularly accurate setting of low post-injection quantities, thus improving setting of the temperature of the particle filter. The method according to the invention is particularly suited for assisting with continuous particle filter regeneration via soot oxidation using NO2, corresponding to the CRT effect.
The injector may be an electromagnetic injector or piezo injector. In the preferred case of an electromagnetic injector, the control current flow through a solenoid, causing the valve needle of the injector to lift up and thus enabling a valve opening for the post-injection of fuel. The duration and height of the needle lift, together with a fuel injection pressure in conjunction with the flow coefficient of the injector, determine the quantity of fuel injected in each post-injection, and thus, the quantity of chemical energy introduced into the exhaust gas that is available for heating after conversion into thermal energy by exothermic oxidation at the oxidation catalytic converter or in the particle filter.
In another embodiment of the invention, above a predefinable temperature of the oxidation catalytic converter, a post-injection quantity is limited in that, during the post-injections, a needle lift amplitude of the valve needle of the injector is set that is less than a maximum needle lift amplitude. In this case the valve needle is not completely brought into its final position in the opened state of the injector, thus allowing small post-injection quantities to be accurately set. For this purpose, a reduced control current is set to a value below a control current nominal value. In this case, the magnetic force generated for the opening motion of the valve needle is less than with control with the nominal current, and the injector opens somewhat more slowly and with a delay. The valve needle typically drops back immediately after reaching the reduced lift amplitude for depositing the post-injection. The duration of a post-injection within a power stroke is thus comparatively short. This is particularly advantageous when the temperature is already high. Thus, the high temperature is easily maintained, and at the same time, due to the short duration, wetting of the wall of the at least one cylinder, operated with post-injection, with post-injected fuel is largely prevented.
In another advantageous variant, a similar behavior results when a duration of control of the injector with the control current is decreased for setting the reduced needle lift amplitude. Below a lower limit control duration, for an injector that is controlled with the nominal value of the control current or also with a lower control current, the valve needle no longer reaches the maximum possible lift, but instead reaches a reduced lift, and after reaching this reduced lift amplitude position quickly drops back to the closed state.
The internal combustion engine according to the invention has an exhaust gas tract in which an oxidation catalytic converter and a particle filter, situated downstream from the oxidation catalytic converter in terms of flow, are situated, and is designed for carrying out a method corresponding to at least one of the variants described above. In particular, appropriate control means such as an electronic control unit and corresponding actuators and sensors are provided for carrying out the control functions.
The features and feature combinations mentioned above in the description as well as the features and feature combinations mentioned below in the description of the figures and/or only shown in the figures may be used not only in the particular stated combination, but also in other combinations or alone without departing from the scope of the invention.